(a) 14 pencils × c450 = c6300;
x erasers × c300 = c6300;
x erasers = c6300/c300; x = 21
He buys 21 erasers costing c300 each with the same amount he uses to buy 14 pencils costing c450 each

(b) Circumference = 44cm; circumference = 2×π×radius; 44 = 2×22/7×radius; 44×7 = 44×radius
Radius = 44×7/44 = 7cm

(c) (2¼ - 1⅝) ÷ 3 7/16 → (9/4 - 13/8) ÷ 55/16 → (18-13/8)×16/55 = 5/8 × 16/55 = 2/11

(d) 5(n+6)/n = 1 → 5(n+6) = n → 5n+30 = n → 5n-n = -30 → 4n = -30 → n = -30/4 = -15/2